Totally up to you. It used to be that you couldn't change it at all after 7 days, but you can change it indefinitely, I think. The only thing you can't do is delete it altogether.

I actually use Steempeak for almost everything on Steem because I can have templates for my posts - which means I can play there and see what it's doing, then just save it for future use. (If you haven't already tried it, it's really simple - replace "steemit" for "steempeak" - and use KeyChain to log in.)